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VM: Virtual Memory or Virtual Mayhem?

How to solve the virtual memory problem in all Linux kernels.

(LinuxWorld) -- The fur has been flying in the Linux kernel development community of late, particularly because there's a lot of contention about how Linux should do virtual memory management (VM). Andrea Arcangeli and Rik van Riel have the two most visible opinions on how the VM should work, so they're often seen as at the heart of the struggle. However, it goes much deeper than that. The VM even seems like a hot button between Linus Torvalds and Alan Cox, although I'm guessing they're not as passionate about the issue as the journalists would like them to be to get mileage out of the topic.

Take us, for example. None other than our own Joshua Drake called the 2.4 Kernel the Kernel of pain, primarily because of problems with the VM algorithms and changes. Yours truly complained a little about the VM last August.

I'm about to weigh in once again about how I feel about VM, only in more detail this time. We're definitely into milking this topic for all it's worth.

After looking at the various VM algorithms in play, I've come up with an alternative. I'm sure Andrea Arcangeli and Rik van Riel are flooded with the same old suggestions over and over again, and it's entirely possible they've heard this one before. Nevertheless, I'll propose it, anyway.

First, let me walk you through a little of the history that gave me the notion of how best to deal with the Linux VM issue.

The brain-dead system that wouldn't die

Back when I was a programmer by trade, one of the projects I worked on involved Fortran programming for a farm of PDP-11s. I honestly don't remember which OS we had installed on the PDPs, but if I had to guess I'd say it was probably RSX-11/M.

What I'll never forget, however, is the one quirk I hated beyond all others. You couldn't run executable files unless they were stored in contiguous blocks on the disk.

Programmers are a virtual disk frag factory, so we ran out of contiguous space almost daily. This means we often found ourselves in the uncomfortable situation where we could no longer test the modifications to our programs until someone defragged the drives. The worst of it was that, if there was such a thing as a disk defrag utility available, the systems guys obviously didn't know about it. They solved the problem by taking the systems off line, backing up, and then restoring the contents of the drives. We couldn't even edit code while we waited.

By the way, Microsoft credits Windows NT architect Dave Cutler as the designer of RSX-11/M (see resources). As tempting as it might be to compare the brain-dead pieces of Windows NT to the bizarre behavior of RSX-11/M, it would be unfair to do so for two reasons. First, more reliable sources than Microsoft only credit Cutler for the design of VMS. Cutler borrowed some RSX-11/M code for VMS, but didn't design RSX-11/M. Second, RSX-11/M was probably only brain-dead because the PDP-11 wasn't much of a brain.

Any computer historian knows the PDP-11 was a breakthrough in affordable computing in its time. Compared to today's desktops, it would be like taking out a second mortgage to buy an abacus. A PDP-11 computer with 8K words of core memory and a 256K disk cost about $30,000 in 1972. It could run you thousands of dollars for an additional 4K of memory.

It may be possible that the RSX-11/M architect was an idiot. It is also possible that the resident portion of the RSX-11/M kernel would have to exceed the size of the typical memory configuration for the PDP-11 in order to change this behavior, which would inflate the price of the system in the process.

I don't happen to recall how much memory we had on our PDP-11s or how much of it was used by RSX-11/M. However, I do know we never had enough. I had to implement overlays to make my Fortran programs work. In case you're not an old fart as I am, overlays are a bit like implementing virtual memory at the application level instead of the OS. You split off parts of your program into modules called overlays. The main program loads an overlay whenever it needs the functions in that overlay.

The trick is to make sure that none of your overlays rely on functions that reside in any other overlays, because your goal is to have only one overlay in memory at a time. I'm not even sure it was possible to have more than one loaded, but it's been too long to remember. Regardless, I'd bet the company paid at least double my yearly salary for some of those PDP-11s, so it was worth the relatively minimal effort for me to break up my program into overlays.

The new VM proposal

What does this have to do with virtual memory performance on the Linux 2.4 kernel?


First, think about what the VM is for. The VM is like an extremely sophisticated automated overlay system, only it deals with many more types of memory storage. It is the part of the OS that comes to the rescue when you have used up all your expensive memory and need more.

Here's how it works. The OS finds some data in memory that can safely be removed and stores it to the cheaper storage (disk swap space), which frees up some expensive memory for other use. When a program needs the data that has been swapped to disk, the OS swaps something else out to disk and brings the needed data back into memory.

Most of the arguments about the VM in Linux revolve around how the OS should decide which memory is swapped and when, and methods to make the process fast and painless.

The remaining arguments are usually about what the OS needs to do when you've filled up all available memory and swap space but some task still needs even more memory. In this case, most people agree that the OS needs to kill one or more running tasks to free up memory. Since you're talking about stopping programs dead in their tracks, you have to address the issue of how the OS decides which tasks are less important than others and can afford to be killed.

If you don't see how this relates to my PDP story yet, then here's a hint. The controversy over the VM involves several extremely talented programmers, each of whom could command hundreds of thousands of dollars per year in salaries. They have been spending a great deal of their time and brain power over the past years figuring out how to squeeze the best performance out of systems with limited RAM and drive space.

Still don't get it? Then let me get right to my proposal for a new VM algorithm for Linux. Granted, this VM algorithm is not meant for typical system loads, but it would solve the most annoying VM problems.

I propose that we create a kernel daemon that checks for either of the following two conditions:

  1. The system swaps to disk so much that you see a severe degradation in performance.
  2. A task needs memory after all available RAM and swap is filled.

If either condition is met, the kernel then kills all tasks except those it needs to display the following message on the screen: "Lay off the doughnuts this week and spend the money to buy another DIMM, you penny pinching skinflint!"

As a bonus, the daemon could check the Internet for current pricing and replace the part about "doughnuts" with some comparison that better represents the current state of the market.

Again, I apologize to Andrea and Rik if this has already been suggested, and I suspect it has. Nevertheless, it was therapeutic, if not useful, to offer the advice.

More Stories By Nicholas Petreley

Nicholas Petreley is a computer consultant and author in Asheville, NC.

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